[GHC] #4370: Bring back monad comprehensions

[GHC]

#4370: Bring back monad comprehensions
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    Reporter:  simonpj           |        Owner:  nsch        
        Type:  feature request   |       Status:  new         
    Priority:  normal            |    Milestone:  7.2.1       
   Component:  Compiler          |      Version:  6.12.3      
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
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Comment(by simonpj):

 Drat, I'd forgotten that the renamer does one step and the typechecker the
 next. That is annoying; I can't see a neat way to do this. Oh well, it's
 not a big deal.  What you have done seems fine.   Please document (in the
 data type declaration) the fact that the 'guard' field is used only for
 monad comprehensions, and is otherwise bottom.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4370#comment:19>
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